1. Field of the Invention
This invention relates to an inverter including a switching circuit for sequentially energizing a plurality of windings of an electric motor such as a brushless motor at a time according to a commutation timing corresponding to a predetermined rotational position of a rotor of the motor and further relates to an air conditioner controlled by such an inverter as mentioned above.
2. Description of the Prior Art
Air conditioners and refrigerators have recently employed, as a compressor motor, a brushless motor classified into a DC motor and an inverter driving the brushless motor for the purpose of variable performance of a compressor or saving electric power consumption. The brushless motor usually necessitates one or more position sensors sensing a rotational position of a rotor of the motor and generating a rotational position signal so that a phase winding to be energized is determined. Since the compressor motor is exposed to the refrigerant in the air conditioners and the refrigerators, it is sometimes difficult to dispose the position sensors in the brushless motor employed in the compressor. In view of this drawback, the present inventor and others have developed a technique for detecting a voltage induced in the motor winding and for electrically processing the detected voltage into the rotational position signal. This technique was applied for patent in Japan and the application was published under Japanese unexamined patent application publication No. 64-8890 (1989).
The above-mentioned technique will be described as the prior art for the present invention with reference to FIGS. 22 to 24. In the following description, the pulse width modulation system is applied to the technique. Referring to FIG. 22 showing the electrical circuit of an inverter, a DC power supply circuit 2 connected to an AC power supply 1 comprises a full-wave rectifier circuit 3, a reactor 4a and a smoothing capacitor 4b. A three-phase bridge circuit 13 serving as a switching circuit is connected between a positive side DC power supply line 5 and a negative side DC power supply line 6 of the DC power supply circuit 2. The three-phase bridge circuit 13 comprises switching elements such as switching transistors 7 to 12. Output terminals 14u, 14v and 14w of the three-phase bridge circuit 13 are connected to terminals of windings 15u, 15v and 15w of a brushless motor 15 respectively. Three transistors 7, 9 and 11 are connected between the positive side DC power supply line 5 and the respective output terminals 14u, 14v and 14 w to thereby serve as positive side switching elements. The other three transistors 8, 10 and 12 are connected between the negative side DC power supply line 6 and the respective output terminals 14u, 14v and 14w to thereby serve as negative side switching elements. When these transistors 7-12 are controlled to be turned on and off in a predetermined order, the windings 15u, 15v and 15w of the brushless motor 15 are repeatedly energized sequentially with a phase difference of 120 degrees (electrical angle), so that the brushless motor is driven. In this case, each transistor is turned on in the period of 120 degrees and off in the period of 240 degrees and furthermore, the duty ratio is controlled in each "on" period by a pulse width modulated (PWM) signal P.sub.1 as shown in FIG. 23(a). Consequently, the terminal voltages V.sub.u, V.sub.v and V.sub.w of the windings 15u, 15v and 15w of the brushless motor 15 have waveforms as shown in FIGS. 23(b), 23(c) and 23(d) respectively.
FIGS. 24(a) and 24(b) show waveforms of the terminal voltage V.sub.u and the winding current I.sub.u of the winding 15u of the brushless motor 15 respectively without the pulse width modulation applied. In the waveform of the terminal voltage V.sub.u, a positive or negative slope section t.sub.a in the electrical angle of 60 degrees represents a voltage induced in the winding 15u and elongated positive and negative pulses represent pulse voltages due to diodes D1 to D6 connected in parallel to the respective transistors 7-12 of the three-phase bridge circuit 13. Reference symbol V.sub.0 represents a reference voltage provided by a resistance type potential divider circuit 16 connected between the DC power supply lines 5, 6. The reference voltage V.sub.0 is set at a half of a voltage in the DC power supply circuit 2 of the three-phase bridge circuit 13. As understood from FIGS. 24(a) and 24(b), a commutation timing lags by about 30 degrees with respect to a time when the induced voltage and the reference voltage V.sub.0 cross, which time will be referred to as "zero crossing time."
The terminal voltages Vu, V.sub.v and V.sub.w are compared with the reference voltage V.sub.0 by respective comparators 18 to 20 provided in a position signal circuit 17 serving as the position sensing means, thereby being converted to fundamental wave signals V.sub.u ', V.sub.v ' and V.sub.w ' for discrimination of 180-degree sections of the terminal voltages V.sub.u, V.sub.v and V.sub.w as shown in FIGS. 23(e), 23(f) and 23(g) respectively. These fundamental wave signals V.sub.u ', V.sub.v ' and V.sub.w ' serve as information about the rotational position of the rotor of the brushless motor 15. The fundamental wave signals V.sub.u ', V.sub.v ' and V.sub.w ' are then supplied to a waveform synthesizing circuit 21 serving as energization signal generating means. The fundamental wave signals V.sub.u ', V.sub.v ' and V.sub.w ' are collated with the PWM signal P.sub.1 by the waveform synthesizing circuit 21 to be converted to recognitive waveform signals U.sub.a, V.sub.a and W.sub.a each comprising continuous square waves composed only of a positive pulse component and having a period of 180 degrees in electrical angle. The recognitive waveform signals U.sub.a, V.sub.a and W.sub.a are out of phase with one another by 120 degrees. A rise point and a fall point of each recognitive waveform signal correspond to the abovementioned zero crossing point.
The waveform synthesizing circuit 21 is provided with first and second timing functions. Six first phase segment patterns X1 to X6 are formed from the three recognitive waveform signals U.sub.a, V.sub.a, W.sub.a by the first timing function. Each of the first phase segment patterns X1-X6 has a period of 60 degrees in electrical angle. Six second phase segment patterns Y1 to Y6 are formed by the second timing function. The second phase segment patterns have start points same as those of the first phase segment patterns X1-X6 respectively and each second phase segment pattern has a period of 30 degrees in electrical angle. The waveform synthesizing circuit 21 finally converts signals of the second phase segment patterns to energization signals U.sub.p, U.sub.n, V.sub.p, V.sub.n, W.sub.p and W.sub.n as shown in FIGS. 23(l) to 23(q) respectively.
The start points of the energization signals correspond to the end points of second phase segment patterns Y1-Y6 and accordingly, lag behind the zero crossing point by 30 degrees. Consequently, the phase patterns of the energization signals correspond to the commutation timing patterns required of the transistors 7-12 of the three-phase switching circuit 13.
On the other hand, a speed determination circuit 22 serving as speed detecting means determines a speed deviation on the basis of a speed command signal S.sub.c and the energization signal W.sub.n supplied thereto from the waveform synthesizing circuit 21 as a speed detection signal representative of the rotational speed of the brushless motor 15. The speed determination circuit 22 generates a speed deviation signal S.sub.d in accordance with the determined speed deviation, which signal is supplied to a pulse width modulation circuit 23. The pulse width modulation circuit 23 controls the duty ratio of the PWM signal P.sub.1 in accordance with the magnitude of the speed deviation signal S.sub.d. The PWM signal P.sub.1 whose duty ratio has been controlled as described above is supplied to gates 25, 27 and 29 of a gate circuit 24 composing drive means. The PWM signal P.sub.1 and the energization signals U.sub.p, V.sub.p and W.sub.p are synthesized by the gates 25, 27, 29 or more specifically, the PWM signal is ANDed with the respective energization signals by the gates, for example, and resultant signals are supplied as base control signals to the bases of the positive side transistors 7, 9 and 11 of the three-phase bridge circuit 13 such that the transistors are on-off controlled in accordance with "on" and "off" modes of the PWM signal P.sub.1. On the other hand, the energization signals U.sub.n, V.sub.n and W.sub.n to which signals the pulse width modulation is not applied are supplied to the bases of the negative side transistors 8, 10 and 12 via gates 26, 28 and 30 respectively so that the transistors 8, 10 and 12 are on-off controlled. Consequently, the transistors 7-12 are on-off controlled by the energization signals U.sub.p, V.sub.p, W.sub.p, U.sub.n, V.sub.n and W.sub.n in the patterns as shown in FIGS. 23(l)-23(q), thereby driving the brushless motor 15. Furthermore, the speed of the brushless motor 15 is controlled by the control of the duty ratio by the PWM signal P.sub.1 as shown in FIG. 23(a).
The above-mentioned "on" mode of the PWM signal P.sub.1 refers either to the high or low level of the pulse signal at which level the transistors are turned on. The transistors are turned on when the pulse signal is at the high level in FIGS. 23(a)-23(q). The "off" mode of the PWM signal P.sub.1 refers either to the high or low level of the pulse signal at which level the transistors are turned off. The transistors are turned off when the pulse signal is at the low level in FIGS. 23(a)-23(q).
As obvious from the foregoing, the windings 15u, 15v and 15w of each phase are energized for the period of 120 degrees with the lag of 30 degrees with respect to the zero crossing time. FIGS. 25(a), 25(b) and 25(c) show the relations among the induced voltage, applied voltage and current without the PWM control in the case of the winding 15u of phase U, for example. The voltage of the DC power supply circuit 2 applied to the winding 15u has a symmetrical waveform about the peak P.sub.t of the induced voltage in the period of 120 degrees. On the other hand, the current I.sub.u flowing into the winding 15u is gradually increased slopewise upon application of the voltage and reaches the normal state with the lag of time period T.sub.1 relative to the applied voltage. The current I.sub.u is gradually decreased slopewise upon completion of application of the voltage, reaching zero with the lag of time period T.sub.2 equal to the time period T.sub.1. Accordingly, the current I.sub.u flowing into the winding 15u takes a waveform unsymmetrical about the peak T.sub.p of the induced voltage, resulting in a phase difference with respect to the induced voltage. This phase difference also occurs when the PWM control is applied.
A torque produced by an electric motor is generally shown by the product of the induced voltage and the current. Since the current lags the induced voltage in the prior art as described above, the efficiency of the motor is reduced. In the air conditioners, particularly, a quick cooling or warming operation is required at its maximum output under the limited power supply capacity. Thus, an improvement in the motor efficiency in the air conditioners or the like has been desired for the energy saving and reduction of the running cost.
In view of the foregoing, it has been proposed that the commutation timing be determined to take a time earlier by a predetermined electrical angle than the time lagging behind 30 degrees the time when the induced voltage and the reference voltage V.sub.0 cross, as shown in FIG. 25(d). The predetermined electrical angle corresponds to a time period T.sub.d in FIG. 25(d). In this case, the waveform of the current I.sub.u is symmetrical about the peak T.sub.p of the induced voltage. Accordingly, since a power factor is improved, the current I.sub.u can be reduced and the motor efficiency can be improved.
However, the above-mentioned lag time periods T.sub.1 and T.sub.2 are not fixed but are varied. For example, the lag time periods become long as the load torque and that is, the current are larger while they are short as the motor speed and that is, the induced voltage are higher. Consequently, a sufficient improvement in the motor efficiency cannot be achieved even when the commutation timing is determined to take a time earlier by the predetermined electrical angle than the time lagging behind 30 degrees the time when the induced voltage and the reference voltage V.sub.0 cross.